Polymers with high melting points, such as high temperature polyamides, are highly valued due to their excellent mechanical properties and chemical resistance. Such high temperature polyamides have found use in automotive parts, electric/electronic components, mechanical components, and many other applications due to the improved thermal stability properties that this type of polymers exhibit.
For example, high temperature polyamides are used extensively in compositions which make possible the production of molded articles with excellent dimensional stability at high temperatures, e.g., in the electrical and electronics industry. Molding compositions of this type are demanded, for example, in the electronics industry for producing components which are mounted on printed circuit boards according to the so-called surface mounting technology, SMT. Components in such applications must withstand temperatures of up to 270° C. for short periods of time without dimensional change and retain very good flame-retardant properties. Very thin-walled components are often needed in the electronics industry because of miniaturization, and a flammability classification according to UL94 of V0 at 0.4 mm is required.
Many known flame retardants, including many halogenated materials, are not suitable under these processing conditions because they are too volatile, not sufficiently thermally stable, have an adverse effect on processing and physical properties, etc. Some flame retardants need to be used at undesirably high concentrations in order to provide the required flame retardant activity.
Salts of phosphorus containing acids are known flame-retardant additives, in particular for thermoplastic polymers. U.S. Pat. No. 3,894,986 discloses flame retardant thermoplastic polyesters containing alkali salts of phosphonic acids, e.g., the mono sodium salt of ethane-phosphonic acid or a sodium salt of a mono-methyl ester of an alkane-phosphonic acid. U.S. Pat. No. 4,972,011 discloses aluminum salts of alkylphosphonic acids or mono-alkyl esters of alkane-phosphonic acids, i.e., salts of compounds of formula (Ia), wherein R is for example methyl, ethyl, propyl or isopropyl etc., unsubstituted or substituted by one or more halo or hydroxy groups; and R′ is hydrogen, methyl, ethyl, propyl, or isopropyl.

DE 3833977 discloses metal salts of compounds of formula (Ia) prepared from reactions of dimethylmethylphosphinate and metal oxides or hydroxides in water at high pressures and temperatures from 120 to 200° C.; reactions run in aqueous solution under elevated pressures at temperatures up to 190° C. in an autoclave are exemplified.
Salts of phosphinic acids, i.e., compounds of formula (II) wherein R1 and R2 are alkyl or carbon based aromatic, are also known flame-retardant additives for thermoplastic polymers.

Salts wherein M is selected from Mg, Ca, Al, Sb, Sn, Ge, Ti, Zn, Fe, Zr, Ce, Bi, U, Na, K or protonated nitrogen base are known. For example, U.S. Pat. Nos. 5,780,534 and 6,013,707 disclose that calcium phosphinates and aluminum phosphinates of Formula (II) are particularly effective in polyester, for example, calcium and aluminum salts of dimethylphosphinic acid, ethylmethylphosphinic acid, diethylphosphinic acid, n-propylmethylphosphinic acid, n-propylethylphosphinic acid, di-n-propylphosphinic acid, diisopropylphosphinic acid or diphenylphosphinic acid.
U.S. Pat. No. 6,255,371 discloses a flame retardant combination comprising, A) a phosphinate of formula (II) above, e.g., a diethyl phosphinate where M is calcium, magnesium, aluminum and/or zinc, and B) condensation or reaction products of melamine e.g., melamine polyphosphate, melam polyphosphate and melem polyphosphate. U.S. Pat. No. 6,547,992 discloses a flame retardant combination for thermoplastic polymers comprising phosphinates and small amounts of inorganic and/or mineral compounds which do not contain nitrogen. WO 2012/045414 discloses a flame retardant composition comprising A) a phosphinic salt of the formula (II) above wherein M is selected from Mg, Ca, Al, Sb, Sn, Ge, Ti, Zn, Fe, Zr, Ce, Bi, Li, Na, K or a protonated nitrogen base; B) a metal salt of phosphorous acid; and other optional components.
U.S. Pub. Pat. Appl. 2009/0030124 discloses flame resistant, semiaromatic, high temperature polyamide resin compositions comprising phosphinate or diphosphinate flame retardant and zinc borate that have reduced corrosion effects on melt processing equipment. U.S. Pat. No. 7,294,661 discloses flame resistant polyamide resin compositions for molded articles comprising aromatic high temperature polyamides with phosphinate and/or diphosphinate flame retardant and, optionally, glass fibers. U.S. Pat. No. 7,723,411 discloses flameproof, high temperature polyamide molding compositions comprising a semi-aromatic, high temperature, partially crystalline polyamide and a flame retardant containing a phosphinic acid salt and/or a diphosphinic acid salt.
U.S. Pub. Pat. Appl. 2011/0021676 discloses that mixtures of phosphinate salts of Al, Mg, Ca, Ti, Zn, or Na with certain metal soaps and metal salts are effective flame retardants in polyesters and polyamides, inclusive of semiaromatic high-temperature polyamides, and exhibit a lower level of wear on materials and higher levels of flowability than the phosphinates of the metals when these are used alone and can be processed at high temperatures without polymer degradation or discoloration. For example, a polyamide composition comprising a mixture of aluminum diethylphosphinate and zinc diethylphosphinate exhibits less corrosion with a simultaneous improvement in flowability when compared to a polyamide composition comprising only aluminum diethylphosphinate
Despite the above disclosures, many of these flame retardants are still not completely suitable for demanding high temperature polyamide applications, such as extrusion and molding of high temperature polyamides. For example, U.S. Pub. Pat. Appl. 20140128516 discloses that polymer compositions comprising many phosphorus acid salts form mold deposits during injection molding, exude during storage under warm, humid conditions, and can give off emissions during compounding. To overcome this difficulty, certain monoarylphosphinic salts are disclosed that when used with selected nitrogen-containing synergists or a phosphorus-nitrogen flame retardant, and optionally with a further stabilizer, are effective flame retardant systems for polymers exhibiting neither polymer degradation nor deposits or exudate.
While many phosphinate and phosphonate salts are said to be thermally stable, this is of course a relative term. As disclosed in US 2007/0029532, decomposition of phosphonic acid salts is well known at temperatures encountered during processing of polyesters and polyamides, damaging the polymers in the process.
U.S. Pat. No. 5,053,148 discloses the preparation of heat resistant foams useful, e.g., as electrical and/or heat insulation materials, obtained by heating polymer compositions, such as polyamide compositions, comprising metal phosphonates or metal phosphonate precursors to temperatures of above 200° C. The foams are seemingly produced by a “foaming process” related to the action, presumably decomposition, of the salts at high temperature. Nothing in U.S. Pat. No. 5,053,148 addresses or refutes the disclosure of US 2007/0029532 that decomposition of such phosphonic acid salts at high temperature gives “brittle compositions which are unusable” as an engineering thermoplastic.
There is still a need for flame retardants with greater efficiency at lower additive concentrations and improved processability for use in preparing flame retardant high temperature polyamide compositions. Copending U.S. patent application Ser. No. 14/337,500 discloses that heating certain alkylphosphonic acid metal salts, such as aluminum salts, calcium salts, zinc salts etc., at temperatures in excess of 200° C. transforms the salts into different materials that are thermally stable at temperatures above 400° C. and can be incorporated onto thermoplastic polymer resins without the adverse impact on physical properties of the polymer seen with many of the salts described above.
It has now been found that the thermally stable products obtained by heating certain alkylphosphonic acid metal salts, such as aluminum salts, calcium salts, zinc salts etc., at temperatures in excess of 200° C. can be incorporated into high temperature polyamides at temperatures above 270° C. without adversely impacting the resulting physical properties of the polymer composition obtained. It has also been found that a lower concentration of the present flame retardants is required to obtain desired flame retardancy in these high temperature polyamides than is needed when incorporated into lower melting polyamides such as nylon 66.